The present invention relates to devices for identifying the metallic composition of a workpiece and more particularly to devices for identifying the metallic composition of a workpiece by measurement of its unique thermoelectric characteristics.
As is well know, when two dissimilar metals are joined in an electric circuit and the two metals are heated or maintained at different temperatures, an electromotive force (EMF) is developed in the circuit in accordance with the so called Seebeck effect. In general, the EMF generated is a function of the temperature and the chemical composition of the particular materials forming the junction.
The Seebeck effect has been utilized in non-invasive (non-destructive) testing of production lots of a specified alloy for quality assurance, for detecting alloys which inadvertently become mixed during a stage of a production process, and for identifying the metal or alloy composition of an unknown workpiece.
Descriptions of such devices are provided in the following U.S. Pat. Nos.: 2,330,599 (Kuehni, 1943); 2,342,029 (Zubko, 1944); 2,366,844 (Doschek, 1945); 2,750,791 (Hanysz et al., 1956); 2,924,771 (Greenberg et al., 1960); 3,093,791 (Richards, 1963); 3,667,032 (Summers Jr., 1972); and 4,156,849 (Rowsey, Snavely, Luce, 1979).
Another example of such a device is the Institut Dr. Forster Tevotest 3.205.
In general, such devices have not provided sufficient resolution and/or repeatable accuracy for distinguishing between alloys with similar compositions. Further, maintaining the test probe at a constant temperature has been critical. For example, the device described in the above noted Summers Jr. patent comprises first and second probes of identical metallic composition. One probe is heated so as to maintain a constant temperature that is elevated with respect to that of the second probe. The two probes are connected in series opposition such that the EMF developed by the cooler probe is subtracted from the higher EMF developed by the heated probe to produce a net EMF indicative of the thermoelectric characteristics of the test piece. As can be readily be appreciated, minor changes in the temperature of the heated probe can cause substantial errors or drift in the net EMF developed, thereby causing an improper test result.
Similarly, such prior art devices are subject to error due to changes in the temperature differential between the heated and cooled electrodes. That problem has been addressed in the above noted Tevotest 3.205. In the Tevotest 3.205 both electrodes are heated to establish a constant temperature therebetween. The temperatures of the probes are continuously monitored through thermocouples.